Method of smelting high quality ferrosilicon

ABSTRACT

An electric furnace operating as a pure resistance furnace in which molten slag layers are retained, carbon is immersed in said slag layers, and raw material is held on the upper portion thereof, immersing its lower portion into the slag layers, whereby direct reducing and indirect reducing (gas reduction) are effected continuously.

United States Patent 1 1 Yonemochi METHOD OF SMELTING HIGH QUALITYFERROSILICON [76] Inventor: Jutaro Yonemochi, Koadachi 171,

Kome-shi, Tokyo, Japan [22] Filed: Nov. 22, 1971 [21] Appl. No.: 201,007

[52] US. Cl 75/12, 13/20, 75/11 [51] Int. Cl C21c 5/52, 1-105b 3/62,C21c 5/52 [58] Field of Search 75/10, 11, 12, 129; 13/9, 33, 20

[56] References Cited I v UNlTED STATES PATENTS I 1,842,536 1/1932BrowneWQ. .L 75/12 2,266,123 12/1941 Kinzel 75/129 3,465,085 9/1969Yonemochi 13/9 Oct. 30, 1973 1,853,544 4/1932 Browne 75/12 2,715,0648/1955 Burns 75/129 2,144,200 l/1939 Rohn 75/12 3,215,522 11/1965Kuhlmann.. 75/11 1,134,127 4/1915 Harrison 75/11 Primary Examiner-L.Dewayne Rutledge Assistant ExaminerPeter D. Rosenberg Attorney-Fidelman,Wolffe & Leitner 5 7] ABSTRACT An electric furnace operating as a pureresistance furnace in which molten slag layers are retained, carbon isimmersed in said slag layers, and raw material is held on the upperportion thereof, immersing its lower portion into the slag layers,whereby direct reducing and indirect reducing (gas reduction) areeffected continuously.

6 Claims, 2 Drawing Figures PAIENIEDncr 30 I915 FIG. 1

IN VENTOR FIG. 2

ATTORNEY METHOD OF SMELTING HIGH QUALITY FERROSILICON The presentinvention is directed to a method of smelting ferrosilicon. Inparticular, the invention is directed to a method of smelting using aresistance furnace in such a manner that both direct and indirectreduction of the raw material occurs at the same time. In the method ofthe present invention the slag layers contain carbon in large quantitiesand the raw material remains in the upper area, generally on top of theslag.

When iron alloys having high silicon content, such as ferrosilicon, aresmelted, a submerged arc furnace is generally employed and electric archeating is utilized. The operation thereof has the followingcharacteristics:

1. Due to the submerged arc furnace for reducing the silica a hightemperature arc, that is, one of more than about 3,000 C. is utilized.2. Raw silica of lower impurity content is used to reduce slagproduction, which amounts to only about 2-6 percent based on the productferrosilicon.

3. When the product is removed from the furnace, it completely flowsout, that is, none of the molten body remains in the furnace.

In an operation method such as described above, the ends of electrodesreach high temperatures, e.g. more than 3,000 C., so that not onlysilicon is reduced from silica. However, other unfavorable reducingreactions also occur during such operations, consuming thermal energyand/or power energy. These reactions produce undesired products, forexample, aluminum is reduced from alumina, resulting in an increase ofimpurities contained in'the product ferrosilicon. In addition, due tothe high temperature, silica (SiO is converted in large amounts to SiOwhich is vaporized, and forms a pulverized dust, thus causing airpollution.

The theoretical temperature to reduce silica to silicon is given by thefollowing formulas:

SiO, (molten) 2C (solid) Si(molten) 2C0 (g 100 percent Si A Z 15923087.17 T

T 1827 K i.e. 1554 C 75 percent Si A Z 145930 88.50 T

. T =1760 K i.e., 1487 C 45 percent Si A Z 154930 88.00 T

T 16490 K i.e. 1376 C The reduction in temperature of silicon, ascompared to Al, Ca, Mg, SiO or like is quite low. Further, in industrialprocessing, in order to increase reaction velocity, temperatures of from150 to 200 above that required to initiate reduction are utilized.However, the melting temperature of silica is 1,670 to l,725 C.Therefore, the silica may not be sufiiciently fluidized at operatingtemperature within the above ranges. Accordingly, surface contactbetween the silica and the carbon present in the furnace may not besufficient to effect significant amounts of reduction of the silica..

The above reaction requires direct contact between the silica and thecarbon, andthis may not actively take place. That is, since the productsare difficult to dispe'rse after the reduction, the reaction does notoften continue.

In the light of the various disadvantages, as mentioned above, relativeto methods of smelting ferrosilicon heretofore, the present invention,as a result of repeated experiments and studies, has been developed andaffords considerable advantages.

' The distinguishing features of the present invention lie in a methodof smelting high quality ferrosilicon wherein the molten slag isaccumulated in the furnace in addition to the raw materials which are acombination of silica (quartz), iron ores, carbon containing material(for use in reduction), and slag making material. The massive amounts ofcarbon are immersed in the slag layer. Due to the buoyance thereof, thecompounded raw material is retained at the upper portion of the furance.Further, at the same time, the ends of electrodes are always immersed inthe molten slag. The ratio of the components in the furance,

Ca0 MgO BaO/SiO should be maintained below about 0.6. By meeting theabove requirements, the temperature can be lowered without lowering theactivity of the SiO,, since the slag layer acts as an electricresistance and the electric current produces Joule heat, by which atemperature necessary and sufficient for reduction of silicon isobtained. Thus the present'invention presents a method of smelting highquality ferrosilicon that allows lower temperature operation withoutlowering the activity of the SiO in the molten slag. The process allowsuse of temperatures of below 1,800 C. in the molten slag.

Accordingly, a primary object of the invention is to provide an electricresistance furnace wherein molten slag is retained in the furnace,electrodes are immersed in the slag, the electrodes acting as anelectric resistance, and electric current is passed so that Joule heatis generated, by which the reduction reaction occurs.

Another object is to provide uniform temperature distribution in theradial direction and to form a rational temperature gradient in theaxial direction within the electric furnace. A further object is toprovide direct reduction in the slag layer of the furnace and preheatingand indirect reduction (gas reduction) in the compounded raw materiallayer.

A further object is to allow use of complex compounds of silica andother oxides, the molten slag therefrom being high in its silicaconcentration and of high activity, which has a melting point close tothe reduction temperature of silica.

Still another object of the invention is the use of an electric powerdensity (350 KW 450 Kw), with the radial area of the molten slag as astandard, such that the slag layer is maintained at a temperature,preferably of from about 1,600" C to l,700 C, and sufficient to easilycarry out the silica reduction.

Thus the ultimate object of the invention is to provide a process forsmelting of ferrosilicon having an extremely high yield, with extremelylow electrode and electric energy consumption.

Other objects and advantages will be obvious from the following detaileddescription of the invention.

Referring to the accompanying drawings, FIG. 1 shows a plan view of afurnace which may be utilized in accord with the present invention.

FIG. 2 is a front elevation of the furnace.

In the drawings, the furnace indicated generally by 11, comprises ashell 13, liner 14, and electrodes 12. In operation, the raw materialsare charged through cover plate 20, and slowly pass downward, the upperraw material containing layer being indicated by the numeral 18, throughthe high slag area, indicated by 17, and the molten ferrosilicon iscollected at the bottom of the furnace 16. During operation productmolten ferrosilicon is withdrawn through passage and the excess moltenslag is withdrawn through passage 19.

In order to. eliminate disadvantages, as described above, the presentinvention is arranged so that when compounded raw material is made bycompounding silica (quartz), iron ores, carbon containing material, andslag making material, complex compounds of silica and other oxides areutilized that will have high concentrations of silica when molten,consequently the activity of the silica containing material is high, andthe melting point of the slag is close to the reducing temperature ofthe silicic acid. 7

Slags, which fulfill the above conditions are, for example, as follows:

The advantageous point of such vheating by resistance, as describedabove, is the ability to form, at will, necessary and sufficient heat,unlike are heat. The inside of the furnace is arranged so that, in thepresent.

invention, if large molten metal layers and molten slag layers arepresent in the furnace, a'uniform temperature distribution may beobtained in the radial direction and a rational temperature gradient maybe maintained in the axial direction in the electric furnace. Therefore,preheating and preliminary reduction is possible in the compound rawmaterial layer.

The density of electric power should 350 KW to 450 KW per square meter,using a horizontal plane of the molten slag layer as the standard. Whenthe silicon component in the product is high, the density of electricpower may well be increased; and when silicon component is low, thedensity of electric power may well be decreased.

With a the determination of the density of electric power and the slagcomposition described above, the temperature of the slag layer may bemaintained at less than l,800 C., preferably in the range of from aboutM.P. im, c.

A molten body comprising the above components is made and large amountsof carbon containing materials are added to the furnace. The-reducingreaction is carten body exceeds 2.5, thus special consideration shouldbe paid to insure immersion of the carbon material in the molten body.

Therefore, in the present invention, the raw material used is made bycompounding quartz, iron ores, carbon containing material and slagmaking material, and a metal layer, a molten slag layer and a solidcompound raw material layer are formed respectively, from the furnacebottom to its top.

The molten slag layer in the present invention is such that theparticles made from the silica, alumina, lime, magnesia or iron oxidecontained in the above compounded raw material is not removed from thefurnace, and, with the massive carbon material, the furnace is filled.The reason being that the amount of carbon containing material decreasesas a result of the reduction and is never molten and, therefore,portions of the carbon material are forced into the molten layer insuccession by the weight of the compounded material. The buoyancy of thecarbon material and the gravity of the compounded raw material locatedat the upper portion are maintained in balanced proportion, thecompounded raw material layer is partially molten, being immersed'in themolten layer, and its melting velocity is maintained at a given value.

The ends of the electrodes are positioned so as to be always immersed inthe molten slag, and thus an electric current passes towards the metallayer accumulated on the furnace bottom effectively making the moltenslag layer a resistance unit. The Joule heat generated at this timeprovides the heat in the furnace and supplies all the thermal energynecessary for the chemical reactions.

l,600 C. to l,700 C., necessary and sufficient for carrying outreduction easily. Thereby, the reaction represented by the formula SiO,2C Si 2C0 is effected on the surface of the carbon containing materialpresent in the molten body, and ferrosilicon is deposited on the furnacebottom in combination with iron formed by eitherv of the followingreactions, FeO +.C Fe CO At the time of the foregoing reactions, thecarbon monoxide generated ascends and as it ascends, moving into thecompounded raw material layer located at the upper portion, gasreduction (indirect reduction) reactions such as:

occur.

In the slag layer, lowering of the concentration of iron relative to thesilicon by reactions of the FeO with reduction and separation of thesilicon, is compensated for by the melting of the compounded rawmaterial, and the necessary concentration thus is always maintained.

The product metal (ferro-silicon) accumulated on the furnace bottom canbe cast, e.g. every hour. The furnace shown being partially (approx.l5-20 percent) extracted in the extractions. The slag formed from theraw material is partially removed in the same relative percentages atthe same time as the product metal, and thus since both metal and slagare only partially removed, the status of inside of the furnace is notgreatly changed. 11

In operation, the size of the charge to the fumacedepends on the furnacesize as well as the rate at which processing can be effected. However,generally charges can vary from about 2,500 kg to about 3,500 kg,preferably about 2,800 kg to 3,200 kg. The charge contains from about 50percent to percent, preferably about 55 percent to 60 percent by weightsilica containing be from about.

material, about 8 percent to about percent, preferably about 9 percentto 12 percent by weight iron ore, and about percent to percent,preferably about 28 percent to 32 percent by weight carbon containingmaterial, the remainder being slag generating material, but suchpercentages, of course, depend on the purity of each of the rawmaterials, and the desired product composition. In addition, the currentutilized will vary in with the speed at which processing is to beeffected, and the composition of the charge.

Finally, the process of the present invention may be utilized to producealmost any ferrosilicon, but is most easily adapted to producingmaterials of from about 30 percent to about 80 percent silicon,preferably about 60 percent to 80 percent silicon, by weight based onthe total product.

EXAMPLE 1 The operation of the present invention is carried outaccording to the method of the present invention using the followingparameters:

Capacity of transformer 25 ,OOOKVA Average power load 20,000KW Terminalvoltage of electric furnace 230V Current 56,000A Power factor ofelectric furnace 89.50% Electric power unit 7,000KWh Silica used l,700KgIron ore 300Kg Slag making material 70Kg Massive carbon material (coke)890Kg Electrodes 25Kg In this operation, the current wave was anextremely satisfactory sine wave, as measured by an oscilloscope andthus the furnace functioned as a resistance furnace. That is, bothelectric power and electrode consumption are extremely low, andferrosilicon of a high silicon content was produced.

What is claimed: 8

l. A method of continuously producing ferrosilicon characterized by lowelectrode and electric energy consumption utilizing an electricresistance furnace, comprising charging said furnace at the upperportion thereof with a carbon containing compounded ferrosiliconproducing mixture, causing said mixture to gradually settle in saidfurnace through a next lower high molten slag containing layer whereinmolten ferrosilicon is produced, causing said produced moltenferrosilicon to settle into a next lower molten product containing area,and withdrawing portions of said product ferrosilicon and said slag atintervals, and maintaining said molten slag at a temperature of up toabout 1,800 C. by charged electrodes, said electrodes being positionedso that their ends are immersed in said slag containing layer, the slagcontaining layer containing calcium oxide, magnesium oxide, bariumoxide, and silica and the weight ratio of said oxides to said silicabeing below 0.6.

2. The process of claim 1 wherein from 15 to 20 percent of said productand said slag are withdrawn at said intervals.

3. The process of claim 1 wherein said slag temperature is maintained atfrom 1,600 to 1,700 C.

4. The process of claim 1 wherein said electrodes are charged so that aradial power density of from 350 up to l.

2. The process of claim 1 wherein from 15 to 20 percent of said productand said slag are withdrawn at said intervals.
 3. The process of claim 1wherein said slag temperature is maintained at from 1,600 to 1,700* C.4. The process of claim 1 wherein said electrodes are charged so that aradial power density of from 350 KW/square meter to 450 KW/square meteris maintained in said slag.
 5. The process of claim 1 wherein the slagproducing material in said charges is selected from the group consistingof Anorthite, Fridimite, and Cristbalite.
 6. The process of claim 1wherein said charge contains carbon containing material of a specificgravity of up to 1.